AIP astronomers have observed the unusual gravititational lens
RBS 825 (also known as SDSS 1004+4112) with the X-ray satellite XMM-Newton.
Gravitational lensing occurs when the observer
sees a background light source almost perfectly aligned with a
massive object in the foreground. Due to bending of the light
by the gravitation of the foreground object, the source can
appear distorted or even be split into multiple images.

Due to gravitational lensing by a foreground galaxy cluster,
the distant quasar RBS 825 is split into 4 images,
separated by up to 14 arcseconds (Figure 1) angular distance.
This is the widest image splitting observed so far and it was possible to
resolve the 4 lensed images with the X-ray telescopes of the XMM-Newton satellite.
Surprisingly the optically brightest image A is relatively faint in X-rays
and hardly visible in the XMM image (Figure 2).
However, gravitational lensing should affect both optical and X-ray light in the same way.
The probable reason for the dimming of image A in X-rays is additional gravitational
lensing by the stars in a galaxy on the line of sight. This so-called
mircolensing seems to zoom into an off-nuclear region of the quasar, but weakens the emission
from the X-ray emitting core.

Optical image of the gravitational lens RBS 825 (Subaru telescope). The yellow circles mark the
4 images of the background quasar. Also visible are the galaxies of the cluster of galaxies,
whose mass is responsible for the lensing effect.

X-ray image of RBS 825 taken with XMM-Newton. The lensed images are marked with circles.
The optically brightest image A is hardly visible in X-rays.